topic_list: Show topic row for empty conversations in left sidebar.

Earlier when a user narrowed to a new topic, there was no highlighted
topic row entry in the left sidebar under the respective stream.

This commit introduces the feature to show highlighted topic in the
left sidebar for a new topic thread. It checks if narrowed topic is
present in the list of topic names to add to it.
It disappears from the left sidebar when unnarrowed and the topic
contains no messages.

Fixes: zulip#22769.

web/src/topic_list_data.ts

@@ -153,6 +153,7 @@ export function get_list_info(
     zoomed: boolean,
     search_term: string,
 ): TopicListInfo {
+    const narrowed_topic = narrow_state.topic();
     const topic_choice_state: TopicChoiceState = {
         items: [],
         topics_selected: 0,
@@ -169,6 +170,15 @@ export function get_list_info(
     const stream_muted = sub.is_muted;
 
     let topic_names = stream_topic_history.get_recent_topic_names(stream_id);
+
+    if (
+        stream_id === narrow_state.stream_id() &&
+        narrowed_topic &&
+        !topic_names.includes(narrowed_topic)
+    ) {
+        topic_names.unshift(narrowed_topic);
+    }
+
     if (zoomed) {
         topic_names = util.filter_by_word_prefix_match(topic_names, search_term, (item) => item);
     }

web/tests/topic_list_data.test.js

@@ -28,6 +28,7 @@ const user_topics = mock_esm("../src/user_topics", {
 });
 const narrow_state = mock_esm("../src/narrow_state", {
     topic() {},
+    stream_id() {},
 });
 
 const stream_data = zrequire("stream_data");
@@ -86,6 +87,28 @@ test("get_list_info w/real stream_topic_history", ({override}) => {
         add_topic_message(topic_name + i, 1000 + i);
     }
 
+    override(narrow_state, "topic", () => "topic 11");
+    override(narrow_state, "stream_id", () => 556);
+
+    list_info = get_list_info();
+    assert.equal(list_info.items.length, 8);
+    assert.equal(list_info.more_topics_unreads, 0);
+    assert.equal(list_info.more_topics_have_unread_mention_messages, false);
+    assert.equal(list_info.num_possible_topics, 11);
+    assert.deepEqual(list_info.items[0], {
+        topic_name: "topic 11",
+        topic_resolved_prefix: "",
+        topic_display_name: "topic 11",
+        unread: 0,
+        is_zero: true,
+        is_muted: false,
+        is_followed: false,
+        is_unmuted_or_followed: false,
+        is_active_topic: true,
+        url: "#narrow/stream/556-general/topic/topic.2011",
+        contains_unread_mention: false,
+    });
+
     override(narrow_state, "topic", () => "topic 6");
 
     list_info = get_list_info();